Fill Gaps between Meshes with smooth surface
It looks like, there are vertices connected with edges and a face on the inside of your model.

This could have happend, when you extruded a face and then cancelled the action. Canceling the action only reverse the move position, but would have given you an extruded face at the exact same position.

You have to remove any unwanted faces/edges and possibly reconnect your vertices in that area. To do that, in Edit Mode, switch to Face Selection Mode (in the menu bar of the 3D View). Then select one face after the other in that area and move them around (G). This way you could easily identify misplaced faces. Remove a face by  RMB selecting it and pressing X > Faces.